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BULK AND SOLUTION
POLYMERIZATIONS REACTORS

Introduction

This article discusses polymerization reactors where the continuous phase is a
solution of a polymer in its own monomer or in a solvent. When the low molecular
weight species is primarily monomer, the reaction is a bulk polymerization; and
when it is a solvent, the reaction is a solution polymerization. This distinction has
little practical importance. The important consideration is that a high viscosity
polymer solution is the continuous phase and is in contact with the reactor walls
and the agitator. In contrast, suspended-phase polymerizations (such as emulsion,
dispersion, and suspension) and gas-phase polymerizations have a low viscosity
continuous phase (see H

ETEROPHASE

P

OLYMERIZATION

).

Table 1 lists representative bulk and solution polymerizations. All these

polymerizations give a polymer-rich continuous phase that, at the end of the
reaction, will have a viscosity 10

4

– 10

7

times higher than the feed solution.

Laminar flow is the norm. Heat and mass transfer coefficients are much lower
than is common in the chemical industry, and specialized equipment is often
necessary.

The manner in which viscosity increases with conversion has a strong in-

fluence on reactor design. Polymerizations with long chain lives, such as conden-
sation and anionic polymerizations increase chain length and viscosity slowly.
When half the monomer has reacted, the number-average chain length will be
about 2 and the viscosity of the mixture will be about twice that of the pure
monomer. Polymerizations with short chain lives, as is typical of free-radical and
transition-metal catalyses, yield high molecular weight polymers from the onset.
A mixture containing 50% monomer by weight might contain a polymer with an

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

Table 1. Representative Bulk and Solution Polymerizations

Heat of

Reaction

Type of

reaction,

exotherm,

By-product

Polymer

polymerization

Process

Reaction medium

kJ/mol

a

K

to be removed

Polyethylene

Vinyl addition

High pressure

Solution of polyethylene in ethylene

95.0

1610

Heat

High density

Polyethylene

Vinyl addition

Solution

Solution of polyethylene in hexane

95.0

1610

Heat

Poly(vinyl

chloride)

Vinyl addition

Bulk

Polymer-rich phase in contact with a

(suspended) vinyl chloride phase

95.8

730

Heat

Polystyrene

Vinyl addition

Bulk

Solution of polystyrene in styrene and

ethyl benzene

69.9

320

Heat

PMMA

Vinyl addition

Bulk

Solution of polymer in methyl methacrylate

56.5

270

Heat

Nylon-6

Ring opening

Bulk

Solution of polymer in caprolactam

15.9

68

None

Polysulfone

Condensation

Solution

Solution of polymers and monomers in

monochlorobenzene with a suspended
NaCl phase

25.1

24

NaCl

Polycarbonate

Condensation

Interfacial

Polymer solution in methylene chloride in

contact with an aqueous NaOH phase

0

0

HCl (as NaCl)

Poly(butylene

terephthalate)

Condensation

Bulk

Polymer solution in diglycol terephthalate

0

0

Ethylene

glycol

a

To convert kJ/mol to kcal/mol, divide by 4.184.

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309

0.5

1.0

0

0

5

10

15

20

25

Conversion

Long chain
lives

Short chain lives

Natural log of viscosity in mPa

s(=cP)

Fig. 1.

Representative curves of viscosity as a function of conversion for short and long

chain lives. The end point in both cases is a number-average chain length of 1000 and 0.1%
residual monomer.

average chain length of 1000 and a viscosity that is 10

4

times that of the monomer.

Figure 1 illustrates the difference between short and long chain lives for a poly-
merization that starts and ends at the same points but proceeds by different
mechanisms.

There are three key design issues in bulk and solution polymerizations:

(1) Removing the heat of polymerization. This is applicable mainly to vinyl

addition polymerizations.

(2) Achieving adequate stoichiometry and by-product removal to obtain high

molecular weight polymers. This is applicable mainly to condensation poly-
mers.

(3) Achieving desired copolymer composition distributions.

Managing the Reaction Exotherm

For vinyl addition and diene polymerizations, heat removal is a primary concern.
The reaction exotherm listed in Table 1 is the heat of polymerization divided by the
specific heat of the polymer. It is a fictitious quantity for energetic polymerizations
since bonds break and the polymer reaches a ceiling temperature well before

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Table 2. Methods for Temperature Control

Method for controlling temperature

Example system

React to low conversion

High pressure polyethylene

Dilute with solvent

Solution polyethylene

Dilute with dead polymer

PMMA casting

Adiabatic reactors

Nylon-6 casting

Boiling CSTRs

Polystyrene bulk continuous

Cold feed to a CSTR

PMMA bulk continuous

CSTRs with heat transfer to jackets

Specialty polymers in small reactors

or internal coils

Flow inside a single tube

High pressure polyethylene

Shell-and-tube reactors

Solution polyethylene

Stirred-tube reactors

Polystyrene bulk continuous

the full exotherm is realized, but it provides a rough measure of the difficulty of
controlling the reaction temperature.

The vinyl addition polymers undergo polymerization by opening a double

bond in a substituted ethylene molecule. The energy release is 50–100 kJ/mol.
The reaction exotherm varies over a broader range since it depends on the mass
of the substituted groups. Thus ethylene and vinyl chloride polymerizations have
the same heat of reaction per mole but very different exotherms due to the rela-
tive mass of the monomers. The vinyl addition polymerizations are too energetic
to allow adiabatic polymerizations, except in very special cases. Heat removal is a
dominant consideration in reactor design. In sharp contrast, a condensation poly-
mer such as poly(ethylene terephthalate) is formed by a series of ester exchange
reactions where the bonds being made and broken have nearly the same energy.
Adiabatic polymerizations are possible for such polymers, and heat may even be
added to evaporate the condensation by-product. Conversion is typically limited
by by-product removal.

Table 2 lists various methods that have been used to control the temperature

in bulk and solution polymerizations. All these processes continued to be practiced
industrially but some would no longer be chosen for new construction.

Reaction to low conversion and the use of large amounts of solvent are con-

ceptually similar. The high pressure process for low density polyethylene limits
conversion to about 15%. This reduces the exotherm to about 250

◦

C. A similar

result is achieved in the solution process for high density polyethylene, where the
conversion of monomer is high but the reactor contains about 85% solvent. Both
processes rely on supplemental cooling by sensible heat transfer to tube walls.
The high pressure uses a single tube and is “once through” with respect to poly-
mer. The solvent process employs a shell-and-tube heat exchanger in a recycle
loop.

Figure 2 shows the reaction exotherm for the semi adiabatic batch poly-

merization of a methyl methacrylate casting system. The methyl methacrylate
monomer has been diluted with dead polymer to limit the exotherm and to in-
crease the viscosity of the casting syrup. The polymerizing mass undergoes a
glass transition and polymerization stops before the temperature reaches the at-
mospheric boiling point of methyl methacrylate, thus avoiding bubbles in the cast

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0

20

40

60

80

100

6

7

8

9

10

Induction
period

Time, min

T

emperature,

°C

Fig. 2.

Reaction exotherm for a methyl methacrylate casting system.

product. A less energetic polymerization such as that for polycaprolactam can be
cast in large parts without need for dilution with dead polymer.

Continuous-flow stirred-tank reactors (CSTRs) can be cooled in three ways.

The most elegant method is to allow boiling of the monomer or solvent so that the
heat of reaction is removed in an overhead condenser. The pressure in the vessel is
set to give the desired temperature. The condensate can be returned to the vessel
or recycled back to the feed. This process is commonly used for polystyrene. Chill-
ing the feed is another means for managing the exotherm in a CSTR. Refrigeration
to

−40

◦

C has been used for the bulk, continuous polymerization of PMMA. Labo-

ratory reactors and small-scale industrial reactors can be cooled using jackets or
internal coils, but this method scales up poorly.

Removing Chemical By-products

Most condensation polymerizations are reversible and become limited by equi-
librium unless the condensation by-product is removed. Spontaneous removal
occurs when the by-product is insoluble in the reaction medium. The low solu-
bility of NaCl in organic solvents means reactions like those for polysulfone and
phenoxy (ie a high molecular epoxy) proceed irreversibly with the chain length de-
pending only on the reaction time and initial stoichiometry. Phenol–formaldehyde
condensations are limited both by the initial stoichiometry and by the formation
of by-product water. Dual limitations are difficult to satisfy when a high molecu-
lar weight polymer is desired, and it is common to change the chemistry so that
the stoichiometric limitation is eliminated. If poly(ethylene terephthalate) were
made by the direct condensation of its two monomers, terephthalic acid and ethy-
lene glycol, exact stoichiometry and removal of by-product water would both be
needed to achieve high molecular weights. The standard industrial process uses

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a preliminary esterification and separation to obtain a self-condensing monomer,
diglycol terephthalate. The monomer has inherently perfect stoichiometry, but its
polymerization is strictly limited by the presence of by-product ethylene gycol.
By-product removal is easy when the viscosity is low. A typical process has one or
two stirred tanks in series that remove ethylene glycol in overhead condensers.
The effluent from the last stirred tank is fed to a specially designed reactor that
approximates piston flow (also known as plug flow). It is typically a twin-screw
device machine that exposes the now viscous polymer solution to vacuum while
moving it forward.

Copolymerization

Random Copolymerization.

Bulk and solution polymerizations are well

suited to copolymerization where a second or third monomer is added to achieve
desired physical properties. Minor amounts of an alpha-olefin, such as 1-butene
or 1-hexene, are copolymerized with ethylene to control crystallinity and den-
sity, producing linear low density polyethylene. A second diol is used to control
crystallinity in poly(ethylene terephthalate). Examples are the two monomers
used in substantially equal quantities, styrene/acrylonitrile polymers and ethy-
lene/propylene rubbers. The need in all these examples is to achieve a uniform
distribution of the various monomers within a polymer chain and between chains
produced at different points or times in the reactor. Batch and tubular reactors
are poorly suited to this task because the monomers react at different rates. When
the chain life is short, molecules formed early and late in the polymerization will
have different average compositions. When the chain life is long, a substantial
composition difference can exist within individual molecules. These problems can
be avoided by using a continuous-flow stirred tank or the functionally equivalent
loop reactor. A loop reactor that consists of a pump, heat exchanger, and recy-
cle piping provides the same mixing environment as a stirred tank but is more
easily scaled up when heat must be transfer through the vessel walls. Boiling
CSTRs also scale well for heat transfer purposes. In either geometry, ie a conven-
tional tank with an internal agitator or the loop reactor, entering feed is rapidly
mixed and distributed throughout the vessel to provide a chemically uniform
environment.

Block Copolymerization.

A polymerization with long chain lives can

be used to make block copolymers (qv). An important commercial example is
styrene/butadiene blocks produced by anionic polymerization (qv). A solution poly-
merization is done in a batch reactor, starting with one of the two monomers.
That monomer is reacted to completion and the second monomer is added while
the catalytic sites on the chains remain active. This produces a block copolymer
of the AB form. Early addition of the second monomer produces a tapered block. If
the second monomer is reacted to completion and replaced by the first monomer,
an ABA triblock is obtained. This process is not easily converted to continuous
operation because polymerizations inside tubes rarely approach the piston-flow
environment that is needed to react one monomer to completion before adding the
second monomer. Designs using static mixers (also known as motionless mixers)
are a possibility.

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Kinetic Models

A process model for any type of polymerization must begin with a kinetic model.
Condensation polymerizations have relatively simple kinetics. The forward re-
action is typically modeled as a second-order reaction with respect to the end
groups, using a single rate constant assumed to be independent of chain length.
The reverse reaction is also modeled as second order. Addition polymerizations
are more complex, consisting of initiation, propagation, chain transfer, and ter-
mination steps. The mathematics are understood but the necessary constants are
rarely published. A significant exception is the classic kinetic study by Hui and
Hamielec (1) on styrene polymerization that, with minor modifications, has had
wide industrial use and serves as a prototype for similar studies.

The gel effect is a special phenomenon encountered in free-radical polymer-

izations, that can have a major effect on reactor design. As the polymerization pro-
ceeds, the viscosity of the reaction mix increases. The diffusivity of all molecular
species decreases, but chain entanglements cause the diffusivity of long polymer
molecules to decrease to a much greater extent than the monomer. The initia-
tion rate is unaffected when a chemical initiator is used and is largely unaffected
even for thermal initiation. Propagation involves a small molecule reacting with
a growing chain and continues at a reasonable rate. However, the termination
step involves interactions between two large molecules so that the termination
rate decreases significantly at high conversion. The concentration of free radicals
increases, as do the total polymerization rate and the average chain length. This
leads to still larger reductions in the termination rate and to an autoaccelerating
reaction that can be almost explosively fast. Poly(methyl methacrylate) provides
the classic example of the gel effect. The autoacceleration shown in Figure 1 is due
to the combined effects of the exotherm and chain entanglements. A commercial
process for polystyrene uses a single stirred tank operated at about 70 wt% poly-
mer. The entire vessel operates in the gel region and thus has a very high volume
productivity.

Tubular Reactors

Polymerizations in tubular reactors have been extensively studied. Most results
are proprietary, although quite comprehensive models for the polymerization of
styrene in a tube are available in the literature. Tubular reactor models combine
the equations for diffusion, reaction, and heat transfer. They are coupled with
the equations of motion owing to the dependence of viscosity on temperature and
composition. The non-Newtonian nature of the polymer solution is of secondary
importance. The resulting set of simultaneous equations is readily solved on a
computer. Results depend on a large number of parameters that are specific to
each polymerization, but the following generalizations are possible:

(1) The low velocities near the tube wall give higher polymer concentrations and

higher viscosities. This further lowers velocities near the wall and increases
velocities near the centerline. In extreme cases, hydrodynamic instabilities

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Thermal
runaway in
this region

Hydrodynamic
instability in this
region

Stable region

Tube diameter, in.

W

all temperature,

°C

0

1

2

3

0

40

20

60

80

100

120

140

160

Fig. 3.

Stability diagram for the thermally initiated bulk polymerization of styrene with

an inlet temperature of 135

◦

C.

lead to oscillations and the jetting of unreacted monomer down the center
of the tube.

(2) The heat of polymerization distorts the temperature profile. When there is

cooling at the walls, the maximum temperature will occur at an intermedi-
ate radial position. For energetic polymerizations, there is a maximum tube
diameter beyond which a runaway becomes inevitable.

(3) Tube-to-tube instabilities can arise when multiple tubes are fed in parallel

from a single pressure source. If extra polymerization should occur in one
tube, the flow through that tube will decrease, leading to still more poly-
merization. When the feed stream is relatively inviscid, it cannot displace
polymer from a tube and the tube plugs.

Figure 3 shows a stability diagram for a bulk styrene polymerization. Per-

formance is governed by four variables: the monomer inlet temperature, the tube
diameter, the wall temperature, and the mean residence time in the tube. Figure 3
shows the effects of wall temperature and tube diameter for plausible values of
the other variables. Very small tubes are stable for any wall temperature. Inter-
mediate size tubes are subject to hydrodynamic instability and large tubes are
subject to thermal runaway. The maximum conversion for a tube larger than 0.95
cm is 20% and is achieved in a 5-cm tube.

The instabilities possible in tubular reactors have had a major influence

on the process design. A model of the high pressure process for polyethylene is
available in the literature (2) and there are even more sophisticated proprietary
models. The highpressure process for polyethylene uses a single tube about 4.5 cm
in diameter and several miles long to avoid thermal runaway and tube-to-tube

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instabilities. Hydrodynamic problems still exist, and some plants have used a
forced oscillation to alleviate them. The discharge valve on the reactor is quickly
opened and closed every few minutes, temporarily lowering the discharge pressure
from about 240 to 69 MPa (35,000 to 10,000 psi) and causing a sudden surge in
flow that redistributes the polymer in the tube.

For most polymerizations starting from monomer, tubular reactors have been

avoided because of the various stability problems. They can be used in recycle
loops where the per-pass conversion is low, in solution polymerizations with large
amounts of solvent, and in post- or finishing reactors intended to drive a polymer-
ization to completion. Shell-and-tube designs with as many as 1000 tubes are used
in polystyrene processes where they also serve as devolatilization preheaters. The
entering polymer solution has a concentration of about 70%, and its viscosity is
high enough to avoid tube-to-tube instabilities.

Kenics-type static mixers have been used as inserts in tubular reactors. Com-

pared to an open tube operated at the same pressure drop, the static mixer gives
about 40% more heat transfer. Stand-alone mixer reactors of the Koch or Sultzer
SMR type have been used as post-reactors and devolatilization preheaters. The
polymer flows through the shell side of the SMR and the heat transfer fluid flows
inside tubes that have been strategically placed to promote radial mixing of the
polymer. One bulk polystyrene process used the SMR as in a recycle loop as the
first reactor, but the capital cost is high compared to alternatives such as a boiling
CSTR or a proprietary stirred-tube reactor.

Some tubular reactors have internal agitation. These include conventional

single- and twin-screw extruders used for grafting (eg maleation) or other modi-
fication reactions that start with a prepolymerized feed. They can provide a sur-
prisingly uniform reaction environment but are extremely expensive in terms of
reactor volume. Economics limit residence times to a few minutes and so they are
rarely used for polymerizations starting from monomer. Specialized designs are
used as polyester finishing reactors, and they are now able to produce the high
molecular weights needed for blow-molded bottles, eliminating the previous need
for solid-state polymerization.

Conveyor belt and moving web reactors give a piston-flow reaction environ-

ment. The polymer layer is usually thin enough so that heat transfer is not a
serious issue, but the confinement of vapors to meet environmental requirements
can be expensive. Thus waterborne or condensation polymerizations without
by-products (eg polyurethanes) are preferred. Ultraviolet and electron beam cur-
ing is common. Applications include photographic and xerographic films, breath-
able fabrics, coated abrasives, and printed films, as well as polymers manufactured
on belt flakers.

Stirred-Tank Reactors

Conventional stirred tanks are commonly used for both batch and continuous
polymerizations. Normal design practice is to use pitched blade turbines, with
impeller to tank diameter ratios of 40–70%, for viscosities up to 10 Pa

·s (100 P)

and sometimes up to 100 Pa

·s. Helical ribbon and sometimes anchor agitators

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are used for viscosities of from 10 to 1000 Pa

·s, and intermeshing, dual im-

pellers are used for the highest viscosities. Jacket heating or cooling is used
for small reactors and for start-ups. Boiling reactors have overhead condensers.
Typically, the condensed monomer or solvent is simply returned to the upper
surface of the liquid, but is sometimes sprayed over the vapor dome to dis-
solve any polymer that might polymerize there. A modern polystyrene process
consists of a CSTR followed by several stirred-tube reactors in series (3). The
stirred-tube reactor is nonboiling and uses internal coils to remove the heat of
polymerization. The design scales poorly, and it seems that these reactors have
reached their maximum size. Productivity increases are achieved by putting sev-
eral of them in series. Other nonboiling reactors use an external circulation loop
with a shell-and-tube heat exchanger in the loop. In some processes, the stirred
tank has been eliminated and the heat exchanger provides most of the reactor
volume.

An important design criterion for a stirred-tank reactor, whether batch or

continuous, is that the mixing time must be short compared to the reaction
half-life. Correlations for mixing time are available in the literature. They range
from a few seconds in laboratory glassware to a few minutes in industrial reactors.
A secondary criterion for continuous-flow stirred tanks is that the circulation rate
must be a large multiple of the net throughput. This criterion ensures that the
residence time distribution approximates the exponential distribution of an ideal
CSTR. A ratio of 10:1 is normally adequate for this purpose. Some loop reactors
have been designed with recycle ratios of 100:1, but this was done to achieve better
heat transfer and shorter mixing times.

Solvent Recovery

Removal of solvent including any unreacted monomer is a necessary step in most
bulk and solvent polymerizations. One possible method is antisolvent coagula-
tion. The polymer solution is contacted with large quantities of another solvent
that is compatible with the original solvent but not with the polymer. When done
correctly, the process gives a suspension of nontacky, microporous particles of poly-
mer that are substantially free of solvent. The particles are collected by filtration
or centrifugation and are further dried. Antisolvent coagulation is the standard
preparative tool of the research chemist but is rarely used industrially because
the separation and recycle streams are so large. It is common for the flow rate
of stream containing the combined solvents to be more than 50 times that of the
polymer stream.

Devolatilization removes volatile components by exposure to heat and vac-

uum. Single- and twin-screw extruders are commonly used for this purpose, and
wiped-film evaporators are used for low volume, specialty polymers. All these de-
vices generate surface area by a combination of mechanical milling and foaming,
but generation of surface area by foaming scales up more easily than mechanical
generation. Flash devolatilization (qv) relies exclusively on foaming and is the
preferred method for high volume polymers.

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BIBLIOGRAPHY

“Bulk Polymerization” in EPST 1st ed., Vol. 2, pp. 642–666, by H. Ringsdorf, Universitat
Marburg; “Bulk Polymerization” in EPSE 2nd ed., Vol. 2, pp. 500–514, by E. B. Nauman,
Rensselaer Polytechnic Institute; “Solution Polymerization” in EPSE, 2nd ed., Vol. 15,
pp. 402–418, by G. Swift and K. A. Hughes, Rohm & Haas.
“E. B. Nauman, Chemical Reactor Design, Optimization and Scaleup, McGraw-Hill, New
York, 2002, Chapts. 8 and 13.

1. A. W. Hiu, and A. E. Hamielac, J. Appl. Poly. Scim. 16, 749 (1972).
2. S. K. Gupta, A. Kumar, and M. V. G. Krishnamurthy, Polym. Eng. Sci. 25, 37–47 (1985).
3. C.-C. Chen, Polym. Eng. Sci. 40, 441–464 (2000).

E. B

RUCE

N

AUMAN

Rensselaer Polytechnic Institute